"At each step of the way, we thought that helping people connect and building this service for people was like a right.

"It was something people should have.

"We thought it was really good. And then you read the press or the blogs about us and it was always like, 'this is like a fad' or 'it couldn't get to the next level' or 'even if it did, it wouldn't be profitable and it wouldn't be business.'"

Zuckerberg says as the anniversary draws near he's been "reflecting a lot on the first 10 years of Facebook."

He says he had no idea that it would become the huge company it is today.

He recalls a conversation with one of his Harvard friends that now works for Facebook running much of the engineering team. (Best guess: Andrew Bosworth.)

After he finished the first version of Facebook for Harvard, he told his friend, "Ok, it's awesome that now there's this utility and community at our school. Clearly, some day someone is going to build this for the world."

He says, "It didn't even occur to me that it could be us. It wasn't even like, maybe it will be us, or hopefully it will be us. I was like, no chance."

He thought it would be built by an established company that already had a bunch of engineers and computers and customers.

"So one of the things I've been reflecting on is why were we the ones?" Zuck says. "And I think the reason was just because we kind of cared more."